According to multipletopics on the Apple Support discussion boards, the problem can occur when a user logs into their Mac's Guest account — whether by accident or on purpose — and then tries to log back into their regular account.

In some cases, users have reported finding their regular account empty of data, as though it were a brand new account.

The current workaround: disable the Guest account. Apple isn't offering any response to this yet, and details are slim, but just to be safe, you may want to at the very least steer clear of your guest account for the time being. [ITWire via Neowin]